Aminoplast resins such as the condensation products of an aldehyde with compounds such as melamine and urea are well known in the art. Additionally, such aminoplast resins which have been etherified to various extents using monohydric alcohols containing from about 1 to 4 carbon atoms, for example hexamethoxymethyl melamine, are also known in the art. Such art known aminoplast resins often are employed, especially in the coatings industry, as crosslinking agents for other thermosetting resins containing active hydrogen atoms as from hydroxyl, amido, and carboxyl groups.
However, a number of disadvantages are attendant upon the use of the known aminoplasts in thermosetting coatings applications. One such disadvantage is undesirable volatilization of the aminoplast with consequent recondensation on the walls of curing ovens under various relatively high temperature, heat curing conditions as for example often exist in curing oil coatings. Often, known aminoplast resins are used to heat cure polyols. It would be desirable if very low molecular weight polyols, such as for example, 1,6-hexanediol, could be utilized in coating compositions which are cured by aminoplast resins, without the many disadvantages that presently militate against their use in such coating compositions. However, when low molecular weight polyols are heat cured with known aminoplast resins, the polyols tend to volatilize from the composition during cure before the crosslinking reaction occurs, thereby giving rise to possible problems of waste due to escape of the volatile materials and possibly undesirable emissions of volatile organic materials to the atmosphere. These problems become increasingly more challenging as the coatings industry attempts to move to the use of coating compositions having higher and higher contents of essentially nonvolatile components.
Another problem arises from the tendency of many low molecular weight polyols to be rather crystalline or solid at room temperature thereby making them incompatible with known aminoplast resins. By incompatible is meant that upon mixing the aminoplast and polyol either do not form a uniform liquid mixture or if they can be made to initially form a uniform liquid mixture as by milding heating, they subsequently phase-separate into a two layer composition.
Additionally, it would be desirable in general, to provide relatively nonvolatile, liquid, low molecular weight thermosetting resins which have viscosities suitable for high-solids coatings applications and yet have a reduced tendency to volatilize during cure.
The present invention is directed to relatively low molecular weight, crosslinkable, resins which can help solve the above problems and to curable compositions, especially coating compositions, containing such resins. These and other objects of the invention will become apparent to one skilled in the art reading the present disclosure.